Numerous golf balls having a multilayer construction wherein the core hardness and cover hardness have been variously improved are disclosed in the prior art. For example, U.S. Pat. No. 6,987,159 to Iwami discloses a solid golf ball with a solid core and a polyurethane cover, wherein the difference in Shore D hardness between a center portion and a surface portion of the solid core is at least 15, the polyurethane cover has a thickness (t) of not more than 1.0 mm and is formed from a cured urethane composition having a Shore D hardness (D) of from 35 to 60, and a product of t and D ranges from 10 to 45.
U.S. Pat. No. 7,175,542 to Watanabe et al. discloses a multi-piece solid golf ball composed of a multilayer core having at least an inner core layer and an outer core layer, one or more cover layers which enclose the core, and numerous dimples formed on a surface of the cover layer. The golf ball is characterized in that the following hardness conditions are satisfied: (1) (JIS-C hardness of cover)-(JIS-C hardness at center of core)≧27, (2) 23≦(JIS-C hardness at surface of core)-(JIS-C hardness at center of core)≦40, and (3) 0.50≦[(deflection amount of entire core)/(deflection amount of inner core layer)]≦0.75.
U.S. Pat. No. 6,679,791 to Watanabe discloses a multi-piece golf ball which includes a rubbery elastic core, a cover having a plurality of dimples on the surface thereof, and at least one intermediate layer between the core and the cover. The intermediate layer is composed of a resin material which is harder than the cover. The elastic core has a hardness which gradually increases radially outward from the center to the surface thereof. The center and surface of the elastic core have a hardness difference of at least 18 JIS-C hardness units.
U.S. Pat. No. 5,782,707 to Yamagishi et al. discloses a three-piece solid golf ball consisting of a solid core, an intermediate layer, and a cover, wherein the hardness is measured by a JIS-C scale hardness meter, the core center hardness is up to 75 degrees, the core surface hardness is up to 85 degrees, the core surface hardness is higher than the core center hardness by 8 to 20 degrees, the intermediate layer hardness is higher than the core surface hardness by at least 5 degrees, and the cover hardness is lower than the intermediate layer hardness by at least 5 degrees.
Additional examples can be found, for example, in U.S. Pat. No. 6,686,436 to Iwami, U.S. Pat. No. 6,786,836 to Higuchi et al., U.S. Pat. No. 7,153,224 to Higuchi et al., and U.S. Pat. No. 7,226,367 to Higuchi et al.
The present invention provides a novel multilayer golf ball construction which may provide one or more of the following benefits: lower spin due to a relatively high core gradient, higher spin on full iron shots due to an outer core surface which is harder than the inner cover surface, and superior overall ball performance properties.